The intensity of the radiation beam used for radiotherapy is required to be time-invariant in some applications or modulated according to beam orientation in other applications, such as Intensity Modulated Radiation Therapy (IMRT).
In IMRT, the delivery can be done using compensators, i.e., filters individually made for each projection, that reduce the intensity to a predefined level in each part of the field due to attenuation of the primary photon beam. However, when using several projections, each projection requiring individual compensators, this technique is time-consuming and requires a lot of effort. The most common way to deliver IMRT is by the use of a multi-leaf collimator (MLC) that has collimator leaves that can be individually positioned to block a small part of the field and thereby shape the beam in the lateral direction into various irregular shapes. In each projection, the collimator leaves are moved during the treatment and thereby various parts of the cross-section of the beam are irradiated during various times, i.e., the dose distribution is modulated.
Currently available IMRT delivery techniques include fixed beam angle delivery and intensity modulated arc therapy (IMAT). When radiation is delivered with fixed beam angles, a series of beam shapes are delivered at each beam angle either dynamically, wherein the leaves of the MLC move during irradiation, or in a step-and-shoot fashion, where the radiation is paused during the movement of MLC leaves. In contrast, IMAT uses multiple overlapping arcs of radiation in order to produce intensity modulation.
While high spatial-frequency modulation can be achieved by individually manufactured physical compensators, low frequency modulation is achieved with standard wedges. A wedge is a device commonly used in radiation therapy to shape the dose distribution from external photon beams. It is available on the radiation therapy machines of all major manufacturers. The most basic form of wedge is the physical wedge, made of metals such as lead or stainless steel. An external physical wedge is mounted outside the machine head. A set of standard wedge angles, typically 15°, 30°, 45°, and 60° are exchangeable. A single internal wedge of 60°, called the ‘universal’ wedge, is also used: the wedge is mounted inside the machine head and wedge angles less than 60° are obtained by combining a 60° wedge field and an open field with weights determined by the desired wedge angle. For example, a 30° equivalent wedge is obtained by irradiating half the time with the 60° wedge and half the time with an open field. Since positioning the wedge in place is slow, the beam is turned off during the wedge motion. The movements of a wedge into in-beam position and subsequently into out-of-beam position are in opposite directions. While the wedge functions properly when stationary, un-compensated radiation would be delivered if radiation is applied during wedge motion.